The present invention relates to control linkages and pertains particularly to linkage means to connect a pair of servo valves of a hydrostatic transmission for simultaneous operation or for individual operation.
Hydrostatic transmissions for track-type vehicles commonly employ a pair of variable displacement, over-center pumps individually hydraulically connected to a pair of drive motors drivingly connected to the tracks. The speed of the vehicle is controlled by selective positioning of the swash plate of the pumps for varying the output displacement while steering is accomplished by reducing the displacement of one pump to decrease the speed of the respective hydraulic motor with respect to the other motor. Thus, the control linkage for controlling pump displacement must be capable of simultaneous control of the swash plate of both pumps in both directions for forward and reverse travel and individual control over the swash plate of both pumps for steering. Furthermore, it is desirable for the control linkage to be returned to a neutral or zero displacement position when the speed input signal is reduced.
One problem with such systems having either one or two pumps, and two motors, is in providing a control system having provision for precise synchronism between the drive motors. The need for synchronism is especially critical when the steering control is in the neutral position for movement of the vehicle in a straight line. Without precise adjustments or position correspondence of the pump and/or motors, the vehicle will not travel in a straight line.
One approach to this problem is disclosed in U.S. Pat. No. 3,772,964 issued Nov. 20, 1973 and assigned to the assignee hereof.